This invention relates to surface mount device packaging. In one aspect, the invention relates to a two-piece package for chip-type electronic parts, the package comprising a carrier tape and a cover tape, while in another aspect, the invention relates to a cover tape designed to dissipate static electricity that may be harmful to the electronic packaged parts. In yet another aspect, the invention relates to a heat-sealable, high clarity, tear resistant cover tape.
Surface mount device (SMD) tape is a two-pan packaging tape for chip-type electronic parts, e.g. integrated circuits, inductors, transistors, resisters, capacitors, diodes, etc. SMD tape comprises a carrier tape with punched or embossed cavities for holding the part and a cover tape adapted to be heat-sealed to the carrier tape. The carrier tape is typically constructed of polyvinyl chloride, polyester, polypropylene or polystyrene, and the cover tape is typically constructed of polyester to which a heat-sealable adhesive is coated onto one side.
In use, the carrier tape and the cover tape are stored on separate rolls or reels. The carrier tape is unwound from its storage reel and extended in a linear fashion such that parts can be inserted into its cavities. As the parts are inserted, the cover tape is applied along the linear length of the carrier tape such that the adhesive coated side of the cover tape comes into contact with the carrier tape. The cover tape and carrier tape are in contact with one another at their linear edges, and the adhesive on the cover tape is activated (rendered tacky) by the application of heat at those points in which it is in contact with the carrier tape. The heat is provided in a sufficient amount, balanced with an appropriate amount of pressure and dwell time, to activate the adhesive such that a bond of uniform strength is obtained across the length of the SMD tape. The heat and pressure can be applied by any one of a number of different techniques, e.g. hot air guns, drag shoes, ultrasonics, reciprocating sealing shoes, heated pinch rollers, etc. The adhesive carried on the cover tape that is not subjected to the heat and pressure is not activated and as such, it remains nontacky.
SMD tapes must possess a number of certain characteristics if they are to be useful as packages for electronic parts. Since most electronic parts are sensitive to static electricity, SMD tapes should be static dissipative (e.g. having a resistivity between about 10.sup.5 and about 10.sup.12 ohms/sq) so that if any static electricity is generated due to friction from contact between the cover tape and the part, then it is dissipated through the SMD tape. The cover tape part of these tapes are also sufficiently transparent to permit any writing (e.g. part numbers, manufacturer's name, etc.) that is borne by most electronic parts to be read through the cover tape.
Peel force is another important property of SMD tapes. Peel force is the force required to remove a cover tape from a carrier tape after the former has been heat-sealed to the latter. If the peel force is too low, e.g. less than 10 g, then the cover tape can loosen from the carrier tape during packing or shipping and the packaged part can be lost. If the peel force is too high, e.g. more than 120 g, then the carrier tape can move or "jump" during the unsealing or "detaping" operation and the packaged part either lost or positioned in such a manner that it is not accessible to a robotic arm programmed to remove it from the carrier tape pocket to its assembly point. The peel force of the SMD tape is the function of a number of different variables including, but not limited to the chemical composition of the adhesive, the method by which the adhesive was activated at the time it was applied to the carrier tape, the conditions to which the SMD tape was subjected from the time of sealing to the time of unsealing, and the amount of time that elapsed between sealing and unsealing
Another aspect of the cover tape that is important to an effective SMD tape is the nature of the adhesive that is coated onto one of its sides. The adhesive must be activated when exposed to sufficient heat, but remain inactive in the absence of such heat. Moreover, only that portion of the adhesive that is subjected to heat and/or pressure should activate, i.e. the lineal edges of cover tape, such that the adhesive at the center of the tape (and over the packaged part) remains nontacky and does not leave a residue on the packaged part should it come in contact with it. Furthermore, the adhesive should not cause blocking when the cover tape is removed from its storage roll for application onto the carrier tape, and it should be sufficiently clear so as not to haze or otherwise reduce the transparency of the backing film to which it is applied such that writing on the packaged parts is obscured. This latter consideration of cover tape clarity is becoming increasingly important in view of the ever decreasing size of the parts, and the movement to laser marking of the parts (laser markings generally providing poor contrast between the part and the marking).
Yet another important aspect of the cover tape is its tear resistance. If the tape tears during a detaping operation on pick and place assembly line, the whole line has to be shut down to replace the defective reel. With the ever increasing speeds of pick and place equipment, the tapes are being subjected to increasing speeds (now in excess of 300 mm/min) of detaping and hence increasing tear forces. As such, the tear resistance property of a cover tape continues to grow in importance.
Various SMD tapes are known (e.g. U.S. Pat. Nos. 5,441,809, 5,346,765, and 5,208,103, all of which are incorporated herein by reference) and commercially available, but all are subject to improvement. Some tapes demonstrate good ability to dissipate static electricity, but the cover tape tends to be hazy. Good dissipation of static electricity generally requires a relatively high loading (more than 30 weight percent) of conductive metal in the adhesive, and this imparts a haze to the optics of the cover tape. Moreover, the backing film of some cover tapes is metallized, i.e. it is coated with a thin layer of metal on one side to impart electrical conductivity to the cover tape, and this reduces light transmission through the cover tape. Some tapes demonstrate relatively good optical properties, but their ability to dissipate static electricity is less than fully desirable, e.g. they possess a resistivity in excess of 10.sup.12 ohms/sq.